Removable lithium battery sizes are made in the format DDLL0. The DD is the diameter, or width; the LL is the length, and the 0 at the end signifies that the battery is round. An 18650 is 18mm wide by 65mm long; a 14500 is 14mm wide by 50mm long; a 16340 is 16mm wide by 34mm long. These numbers represent the size of the electrical cell from which the battery is made, so the wrapper/label makes them a wee bit wider and protection circuitry, buttons, end plates &c. can make them anywhere from a wee bit to a few mm longer. Not all batteries that are the "same size" are actually the same size. 18650's can be anywhere from about 64mm to just over 67mm long and can vary from 18mm to almost 19mm wide. Conversely, some batteries that look like they are different sizes aren't, really, when you actually look at the sizing. 18500's and 18490's are, for the most part, interchangeable, as are 17650's and 17670's. This is because the size differences of these "models" are less than the variation in sizes of the same "model" from one brand to another.

MaH, or milli-amp hours, is the "amount" of electricity each battery can hold. The MaH is a measurement of how long the battery can last at a specific discharge rate. This discharge rate is determined by the resistance of the circuit (which is primarily the coil resistance, for our purposes) but it will also be affected by the voltage at which a PV is operated if it is a variable-voltage device.

The C rating of a battery is its discharge rate. It is not generally printed on the battery label and it can be difficult to find the specific C rating for any given battery. A 2C battery can put out twice its MaH rating "safely" while a 10C battery can discharge ten times its MaH rating. Most people, though, only need to know that "low drain" batteries are fine for vaping at their stated voltage with standard resistance coils. The lower the resistance of the coils and/or the higher the voltage at which your PV operates, though, the higher the rate of discharge will be. "High drain"/"high discharge" batteries will work for anything, but low drain/discharge batteries may overheat under certain conditions.

LiIon batteries are prone to failure if they are not protected. Since most PV's do not have built-in protection, you should only buy "LiIon" or "lithium ion" batteries that are protected. The protection circuitry built into the battery adds about 2mm to the length, so a "protected" 18650 is usually around 67mm long and a "protected" 14500 is usually around 52mm long.

IMR batteries are also "lithium ion" batteries, but manganese is used instead of the cobalt in plain LiIons. This means that it cannot hold quite as much charge (so the MaH is lower), it has much less internal resistance (so the discharge rate is higher), and if it fails it is much less likely to fail in a violent manner. Because of the lower likelihood of failure (due to the higher discharge rate) and the fact that failure will be hot gas rather than flames and/or explosion, IMR's are known as "safer chemistry" batteries and generally do not have a protection circuit added. This means an IMR 18650 is actually very close to 65mm long.

LiFePO4's are also "lithium ion" batteries, but ferric phosphate is used instead of cobalt or manganese. This gives them a much higher discharge rate than other LiIon's. The biggest difference for our concerns, though, is they are considered a "safer chemistry" battery like the IMR's but that the chemical matrix stores the electricity in such a way that their nominal voltage is 3.0 volts rather than 3.7 volts.

Purpose-built batteries for vaping (the ones where you have to screw the charger into the socket that holds the atomizer or cartomizer) are all protected LiIons. They are also made differently and often for specific purposes or vaping styles and those differences are well beyond the scope of this little intro...